The invention relates to a process for the ink control of a printing machine with colorimetric ink control regulation, wherein on the printed sheet, color measuring strips with several color measuring fields are also printed. The color measuring strips are optically scanned by means of a measuring head in order to determine the spectral intensity distributions of the color measuring fields to thereby determine the spectral reflections and the color location of a reference field for the color measuring fields in a color coordinate system from a spectral color analysis of the measuring light, and to produce, by coordinate comparison, a control variable for the adjustment of the ink control elements of the printing machine using the color deviation of the color measuring field scanned, so that undesirable color deviations on the sheets subsequently printed with the new ink control settings will become minimal. This invention also relates to an apparatus for carrying out the process, the apparatus having at least one measuring head connected with a spectrometer to scan the co-printed color measuring fields, and a measured data processing unit for processing the measured data of the spectrometer into adjusting values for the printing machine.
A process of the aforementioned type is known from EP A No. 228 347, in which for the optimal adaptation of the color imprint, a plurality of color measuring fields are evaluated as reference fields, in order to obtain a proof having an optimal adaptation of the color imprint of sensitive locations of the print important for the image, whereupon in production printing, ink controls regulated by ink density may be superimposed on the ink controls regulated by color deviation. The spectral color analysis of a plurality of color measuring fields and the calculation of a multitude of color coordinates for every printed sheet requires a relatively high effort. This effort is further increased because deviation vectors are weighted for each of the numerous reference fields in the known process, in order to minimize the overall color deviation determined as a measure of quality from the amounts of the individual color deviations. To determine the layer thickness variations associated with the individual color measuring fields, it is necessary to multiply the color deviations vector correlated with the numerous reference fields with empirically determined conversion matrices. The empirical determination and storage of the numerous conversion matrices alone already represents a very great effort. As the printing ink components involved are corrected more or less independently of each other, an unfavorable convergence behavior is obtained. For this reason, the known process proposes to use separate color measuring fields for particularly critical color measuring fields, which again increases the overall effort.